After you remove the shell you will see a DC lighting board like the one above.
Slide the lighting board towards the back of the locomotive and lift to remove it.

Be sure to apply an insulator (here we used Kapton tape, but eletrical tape would work as well) over the frame to prevent the motor tabs from shorting on the frame.
Reverse the motion you used to remove the lighting board to install the decoder. Make sure that the copper tabs on the decoder cover the motor tabs in the locomotive.

Above photo shows completed installation. This is a very easy board replacement installation.

Important Soldering Tip

Please do not use any flux either liquid or paste on the mother board. It will damage it. Use only Rosin core solder approved for electronics use.
We recommend to use only Kester "44" rosin core, SN63PB37, .015" diameter, part number 24-6337-0007.
This can be ordered from Techni-Tool under Techni-Tool part number 488SO157http://www.techni-tool.com

Other solder tips

When stripping wire, only strip a tiny little bit of the insulation. Strip no more then a 1/64 of an inch. When the wire gets tinned with solder, the insulation will shrink back more. Try to not expose any more wire then half the length of the solder pad at most. In no case should solder or exposed wire wire ever be outside the boundary of the the solder pad you are attaching a wire to.Click here for important information on properly Stripping and Tinning wire